Electronic and magnetic properties of carbon-, sulfur-, and non-doped ZnO clusters are investigated using density functional theory (DFT). It is found that in the case of non-doped systems a planar graphitic-like reconstruction is necessary for the observation of a magnetic moment; carbon impurities could promote ferromagnetism; and sulfur impurities at the surface of ZnO nanoparticles hamper any magnetic state and open the HOMO-LUMO gap. Our findings suggest that the experimentally observed magnetic properties of ZnO systems are driven by competition in the structural conformation and are extremely sensitive to the surface and interfaces. (C) 2010 Elsevier B.V. All rights reserved.